The invention is based on a priority application EP 03 360 006.5 which is hereby incorporated by reference.
The present invention relates to a Raman amplifier comprising at least one length of Raman amplifying fiber and at least a coupler for coupling at least a first pump laser module and a second pump laser module to said Raman amplifying fiber, the first pump laser module comprising a frequency discriminator for selecting an optical frequency to be emitted with an optical power exceeding an optical power of remaining optical frequencies that are also emitted by said first pump laser module.
Further, the present invention relates to a method of pumping a Raman amplifier comprising the steps of providing at least one length of Raman amplifying fiber, coupling at least a first pump laser module and a second pump laser module to said Raman amplifying fiber and, selecting an optical frequency to be emitted by the first pump laser module with an optical power exceeding an optical power of remaining optical frequencies that are also emitted by said first pump laser module.
Such a Raman amplifier and such a method are known from U.S. Pat. No. 6,384,962 and from U.S. Pat. No. 6,320,884.
In general, Raman amplifiers are utilized for amplifying optical signals in wavelength division multiplexing (WDM) long distance optical fiber communications systems.
Light propagating in such systems is prone to attenuation. Optical amplifiers help to compensate for such attenuation by providing additional power to the optical signal as it propagates through the system.
A Raman amplifier provides for pump light that is introduced along a length of the optical fiber that guides the optical signal. The pump light wavelength is shorter than the signal wavelength. Accordingly, pump light photon energy exceeds signal photon energy. Energy is transferred from the pump light to the signal by stimulated Raman scattering. Utilizing this physical effect for signal amplification is per se known.
In a simplified depiction, pump light photons of high energy are absorbed by scattering particles such as SiO—2 and/or GeO—2 inside the fiber due to inelastic scattering processes. As a result, the scattering particles are excited, i.e. their energy is increased to a higher energy level Ee as compared to the energy E0 of the non-excited state. If the excited molecule does not return to its non-excited state but to an excited state of lower energy E1 (E0<E1<Ee), a (first) photon of Energy E_s with E_s=Ee−E1 is emitted. The photon may be emitted spontaneously or the emission may be stimulated by a further (second) photon or signal photon of energy E_s propagating through the fiber. If the second photon forms part of an optical signal, i.e the second photon is a signal photon, the optical signal is, thereby, amplified by stimulated emission or, stated otherwise, by stimulated Raman scattering.
Stimulated Raman scattering gives rise to an amplification of the incoming optical signal when the signal frequency is shifted from the frequency of the pump light by the Stokes frequency. The Stokes frequency is a characteristic of the fiber material and does not vary with varying pump light frequency. Due to the impact of the solid state material embedding the scattering molecules in the fiber, the gain spectrum of monochromatic pump light is continuously distributed over a wavelength spectral range of approximately 20 nm. In other words: The phenomenon that light of a frequency f propagating in a material generates light of the frequency f+/−delta_f is called Raman effect. The light of frequency f−delta_f is called Stokes light, and the light of frequency f+delta_f is called anti-Stokes light. An optical direct amplification is carried out by using the stimulated Raman scattering phenomenon which is a non-linear effect of an optical fiber.
Among the different available pump light sources are semi-conductor laser diodes. These laser diodes generally show a spectral emission characteristic that is broad due to specific semi-conductor properties. In terms of wavelengths, the value of 30 nm represents a typical curve bandwidth.
For this type of pump light sources, lasing at a specific optical frequency can be achieved by adding a Fiber Bragg Grating (FBG) to the source, for instance in the pigtail fiber of the chip. The use of a Fiber Bragg Grating allows easy selection of a narrow optical frequency range emitted by the pump diode and stabilizes emitted optical power. Utilizing Fiber Bragg Gratings for wavelength selection is per se known. In a simplified depiction, a Fiber Bragg Grating may considered as a periodic structure of refractive index variations in a light guiding portion of the optical fiber that can reflect light of a certain wavelength propagating along the fiber. The periodic structures may be generated by exposing a doped fiber to structured ultraviolet radiation. The reflected light propagates in the fiber in a direction opposite to that of the incident light. If a diode laser is pigtailed to a fiber containing a Fiber Bragg grating, and if the centre of the grating bandwidth is within the gain bandwidth of the laser, then the optical spectrum of the diode laser will be affected. Pump light waves of a certain wavelength are reflected at a plurality of refractive index variations that are spaced apart by half the wavelength. The reflected waves interfere constructively and result in a respective intensity peak at the particular wavelength. The Intensity peak's bandwidth is much smaller than the original emission curve of the laser diode. Accordingly, a Fiber Bragg Grating represents a wavelength selective reflector.
Since the Raman amplifier is illuminated by this pump, a large part of the incident pump light energy is concentrated on the spectral bandwidth of the Fiber Bragg Grating output.
As a consequence, the output of the Raman amplifier is also concentrated on a limited bandwidth, usually to a 1-dB bandwidth of approximately 40 nm.
It is, however, desirable to have a broader signal bandwidth of Raman amplifier output in order to be able to amplify a broader bandwidth of optical signals.
To achieve a broader bandwidth, the above mentioned U.S. Pat. No. 6,384,962 discloses the utilization of multiple pumps, i.e. at least a first pump and a second pump, the first and the second pumps producing gain curves with respective maxima and minima, wherein the maximum of the second pump related curve coincides with a minimum of the first pump one. In this way, an uneven composite gain signal (gain ripple) is said to be equalized. However, due to the nature of adding independently generated gain curves, the resulting gain curve is still prone to show gain ripple. The equalization actually achieved is a function of the number of pumps. The more gain curves are superposed, the better equalization is achieved. In other words: According to U.S. Pat. No. 6,384,962, at least two independently generated gain curves are superposed, however, without major constructive interaction in the process of generating both gain curves.